This invention relates to an x-ray monochromator assembly for focussing a monochromatic beam of diffracted x-radiation at a detector.
The diffracted beam monochromator is a particularly useful attachment to the routine powder diffractometer due to its ability to remove scattered primary white radiation and specimen fluorescence. Although, when used with suitable apertures, the band pass of the graphite crystals typically employed in these monochromators is sufficient to reject diffracted radiation, they are not good enough to resolve, for example CuK .alpha..sub.1 from CuK .alpha..sub.2. The major advantage of graphite crystals over crystals such as LiF (200) or quartz (1011) lies in its high diffraction efficiency. This high efficiency stems from the extremely mosaic nature of pyrolytic graphite which mosaicity also is the reason for the rather wide band pass. (See "Introduction To X-Ray Spectrometry" - by Ronald Jenkins, Heyden, London, 1976, page 84). Thus, diffracted beam monochromators employing graphite crystals are typically considered as means of partial monochromatization.
Before the advent of the graphite crystal, monochromators were generally supplied with a LiF (200) crystal. Use of such a monochromator causes a loss of some 80% of the intensity of the characteristic diffracted beam and consequently every attempt was made to reduce further loss of efficiency of the device. As an example, no collimator was employed between specimen and detector. The lack of such collimation causes some deterioration of the diffracted beam profile distribution due to the increased axial divergence of the beam. Earlier monochromators were also provided with the capability of working with different wavelengths and suitable adjustments would allow their use with most of the target materials used in diffractometry. With the modern trend to the use of high specific intensity, fine-focus copper anode tubes for most routine applications in inorganic and mineral analysis, the need for this versatility with its associated mechanical constraints has diminished.
Although the use of the collimator results in a highly improved profile shape of the diffracted beam, still some loss in beam intensity occurs thus resulting in reduced counting rate efficiency when it is employed.
Since under some circumstances, good profile shape rather than counting rate efficiency is important while under other circumstances counting rate is more important it is desirable to have a monochromator in which the collimator may be moved in and out of position in the path of the diffracted beam without changing the position of the crystal monochromator.
A special problem in the alignment of this type of diffractometer configuration is the accurate setting of the specimen to receiving slit distance, which adjustment is best done with the x-ray path energized.